Electrophilic Addition Reactions (DP IB Chemistry)

Electrophiles

What is an electrophile?

• An electrophile is a species that forms a covalent bond when reacted with a nucleophile by accepting electrons

• They are electron-deficient so will have a positive charge or partial positive charge

Examples of neutral and charged electrophiles

Electrophilic Addition Reactions

• Electrophilic addition is the addition of an electrophile to an alkene double bond, C=C

• The alkene double bond, C=C, is an area of high electron density which makes it susceptible to attack by electrophiles

• The C=C bond breaks forming a single C-C bond and 2 new bonds from each of the two carbon atoms

• Electrophilic addition reactions include the addition of:
  

  • Steam, H₂O (g) to form alcohols

  • Hydrogen halides, HX , to form halogenoalkanes

  • Halogens, X₂, to form dihalogenoalaknes

Why does the C=C bond react with electrophiles?

• Alkenes are unsaturated molecules that contain a C=C bond

• The atoms around the carbon-carbon double bond adopt a planar arrangement and the bond angle is 120^o

Diagram to show the planar arrangement of the C=C bond

The bond angles are 120^o

• The presence of the C=C bond gives alkenes a number of chemical properties that are not seen in alkanes

• Since the alkene contains π-bonds, it is possible to break the weaker π-bond and form stronger  σ-bonds with other species without forcing any atoms on the molecule to break off

• As a result alkenes (unlike alkanes) are capable of undergoing addition reactions

• The ability of alkenes to undergo addition means that they are much more reactive than alkanes

Diagram to show the general equation for addition reactions across the C=C

Addition reactions in alkenes

Addition of water

• When alkenes are treated with steam at 300 ^oC, a pressure of 60 atmospheres and sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄) catalyst, the water is added across the double bond in a reaction known as hydration

• An alkene is converted into an alcohol

• The reaction processes via an intermediate in which H⁺ and HSO₄^-  ions are added across the double bond

• The intermediate is quickly hydrolysed by water, reforming the sulfuric acid

• The following equation shows the conversion of ethene to ethanol

CH₂CH₂  CH₃CH₂OH

ethene               ethanol

• This is a very important industrial reaction for producing large quantities of ethanol, a widely used solvent and fuel

• The process is much faster and higher yielding that producing ethanol by fermentation

Addition of halogens

• The reaction between alkenes and halogens is known as halogenation

• It is an example of an electrophilic addition where an electrophile ('electron seeker') joins onto to a double bond

• The C=C double bond is broken, and a new single bond is formed from each of the two carbon atoms

• The result of this reaction is a dihalogenoalkane

This reaction occurs readily at room temperature and is the basis for the test for unsaturation in molecules

Halogenation in alkenes

• Halogens can be used to test if a molecule is unsaturated (i.e. contain a double bond)

• Br₂ is an orange or yellow solution, called bromine water

• The unknown compound is shaken with the bromine water

• If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

Diagram to show the colour change that occurs when testing for unsaturation

The bromine water test is the standard test for unsaturation in alkenes

Addition of hydrogen halides

• Alkenes will react readily with hydrogen halides such as HCl and HBr to produce halogenoalkanes

• This reaction is known as hydrohalogenation 

• It is also an electrophilic addition reaction that occurs quickly at room temperature

Formation of a halogenoalkane from an alkene and hydrogen halide

Hydrohalogenation reactions in alkenes

• All the hydrogen halides react in this way, but the fastest reaction occurs in the order HI > HBr > HCl due to the increasing bond strength of the hydrogen-halogen bond, so the weakest bond reacts most easily